1. Field of the Invention
This invention relates to a non-aqueous electrolyte for a battery and a non-aqueous electrolyte battery comprising the same, and more particularly to a non-aqueous electrolyte for a battery having a non-combustibility and a non-aqueous electrolyte battery having excellent battery performances and a high safety.
2. Description of the Related Art
The non-aqueous electrolyte is used as an electrolyte for a lithium battery, a lithium ion secondary battery, an electric double layer capacitor or the like. These devices have a high voltage and a high energy density, so that they are widely used as a driving power source for personal computers, mobile phones and the like. As the non-aqueous electrolyte are commonly used ones obtained by dissolving a support salt such as LiPF6 or the like in an aprotic organic solvent such as an ester compound, an ether compound or the like. However, since the aprotic organic solvent is combustible, when it leaks from the device, there is a possibility of firing-burning and also there is a problem in view of the safety.
As to this problem is examined a method for rendering the non-aqueous electrolyte into a flame retardance. For example, there are proposed a method wherein a phosphate such as trimethyl phosphate or the like is used in the non-aqueous electrolyte, and a method wherein the phosphate is added to the aprotic organic solvent (see JP-A-H4-184870, JP-A-H8-22839 and JP-A-2000-182669). However, these phosphates are gradually reduction-decomposed on a negative electrode by repetition of discharge and recharge, so that there is a problem that battery performances such as discharge-recharge efficiency, cyclability and the like are largely deteriorated. Also, when the battery using the phosphate is stored at a charged state, the decomposition reaction of the phosphate progresses due to a high voltage of the battery even if the battery is not discharged and recharged, so that there is a problem that the battery performances after the storing are largely deteriorated.
As to the latter problem, there are attempted a method wherein a compound for suppressing the decomposition of the phosphate is further added to the non-aqueous electrolyte, a method wherein the molecular structure of the phosphate itself is devised, and so on (see JP-A-H11-67267, JP-A-H10-189040 and JP-A-2003-109659). Even in these methods, however, there is a limit in the addition amount and also the flame retardance of the phosphate itself is deteriorated and the like, so that the electrolyte gets only into the self-extinguishing property and the safety of the electrolyte cannot be sufficiently ensured.
Also, JP-A-H06-13108 discloses a method wherein a phosphazene compound is added to the non-aqueous electrolyte for giving the flame retardance to the non-aqueous electrolyte. Some of the phosphazene compounds exhibit a high non-combustibility and have a tendency to improve the flame retardance of the non-aqueous electrolyte as the amount added to the non-aqueous electrolyte is increased. However, since the phosphazene compound exhibiting the high non-combustibility is generally low in the solubility of a support salt and the dielectric constant, as the addition amount is increased, the precipitation of the support salt and the lowering of electric conductivity are caused, and hence the discharge capacity of the battery may be lowered or the discharge-recharge performance may be deteriorated. Therefore, when the phosphazene compound exhibiting the high non-combustibility is added, there is a problem that the addition amount is limited.
Furthermore, JP-A-2006-107910 proposes a non-aqueous electrolyte comprising a combination of a fluorinated phosphate and a phosphazene compound as a technique for simultaneously establishing the high non-combustibility and battery performances. The non-aqueous electrolyte is high in the flame retardance and excellent in the battery performances, but when it is exposed to a severe condition such as high temperature or the like at a charged state or when discharge-recharge are repeated at a low voltage, the capacity tends to gradually decrease just the same.
Recently, the devices such as lithium battery, lithium ion secondary battery, electric double layer capacitor and the like are also actively developed as a power source in vehicles including HEV. For such an application, it is required that the safety is high and stable performances can be ensured within a wider temperature range or wider voltage range, but the conventional techniques can not be said to have a satisfactory level in these points.